Laser imaging is well known. In a typical laser imaging process a laser-sensitive media is mounted onto the surface of an imaging cylinder and an image is imparted onto the media using a focussed write laser. FIG. 1 shows a prior art external drum imaging system 1 having an imaging head 2 directing a laser beam or beams 18 towards a media 4. The media 4 is rotated on a drum 5 while the imaging head 2 is translated along the drum by a lead screw 6, thus scanning or writing a series of bands or a helical pattern around the drum.
Alternatively, the scanning motion can be generated using a flatbed imaging system where the media is held on a platen and relative motion is generated in two orthogonal axes between the media surface and the imaging beam. The imaging systems described are used in dedicated devices for imaging many different kinds of media including lithographic plates, flexographic plates, screens for screen printing, gravure cylinders as well as layers for flat panel displays, printed circuit boards and the like. Furthermore, the imaging system could be incorporated directly on a printing press for imaging plates in situ. Since such systems are well known in the art they will not be further discussed herein.
During imaging, the interaction of the laser and the media causes a physical and/or chemical change to the imaged areas of the media. In the process of imaging, matter may be expelled from the laser sensitive media. The expulsion of matter from the media is referred to as ablation. The matter expelled may include solids, liquids, gases, or plasma, or a combination thereof, more commonly referred to by the terms “smoke” or “particulate debris”. Ablative media are imaged by selectively dislodging or evaporating material from a layer of the media to form an image. While ablative media by nature produce ablation debris, media traditionally regarded as non-ablative can also produce some smoke fumes and/or particle debris, particularly when imaged by high power lasers such debris can also be termed “ablation debris”.
Ablation debris presents several difficulties, which may hamper the imaging process. A first problem is that the debris may obstruct the laser beam thus affecting the imaging of the media. Ablation debris can also resettle onto the media; this is known as redeposit. Redeposit is a particularly critical problem in imaging laser sensitive media, because redeposit can cause imaging artifacts that may be visible on the final product. Once redeposit has occurred it is difficult to remove without damaging the imaged media. A third problem associated with ablation debris is related to its tendency to accumulate in the sensitive areas of the imaging lasers and other areas of the imaging device. Accumulation of ablation debris can cause severe degradation and/or damage to the components in the imaging system, particularly the laser optics. For example, if a layer of debris collects on a lens, it may drastically affect the lens' optical performance. Furthermore, the danger of ablation debris is not limited to optical degradation, since some media have partially conductive material compositions. Ablation debris from such materials can cause failures in electrical and electronic systems if it is released into the machine environment during imaging.
The escapement and subsequent accumulation of debris over a long time represents a maintenance cost related to cleaning the affected components. The issue is particularly relevant to the problem of build-up on the optical surfaces, since these components are delicate and difficult to clean and may require the dispatch of a specially trained service person to perform maintenance. Furthermore, as customers become increasingly demanding in respect of image quality, the tolerance for even slight degradation due to ablation debris on the optical elements is substantially reduced. If the time between cleaning of the optical components can be extended this represents a significant reduction in downtime and maintenance cost for the imaging system.
There is a need for better methods and apparatus for reducing the accumulation of ablation debris on optical elements in imaging devices.